CT systems produce planar images along imaginary cuts through a subject. Each cut is referred to as a slice. Scanners in general comprise an X-ray source which revolves about an imaginary axis through a subject. The X-rays, after passing through the subject, impinge on an opposing array of detectors, which may also be revolving. Data for reconstruction of a single image includes a set of views corresponding to different projection angles, each view comprising radiation intensity data measured by detector elements at a particular projection angle.
A prior art CT study of a subject for obtaining successive planar images includes the following steps:
1. Putting the patient on the bed in a CT system gantry. PA1 2. Scanning the patient. The scan includes the revolution of the X-ray source about the subject and acquisition of radiation intensity data per detector element per angle of revolution of the X-ray source. PA1 3. Reconstructing the image. Optional display, archiving and/or filming. PA1 4. Incrementing the bed to the position of the next slice. PA1 1. P. Slavin, U.S. Pat. No. 3,432,657 (1969). PA1 2. I. Mori, U.S. Pat. No. 4,630,202 (1986). PA1 3. H. Nishimura, U.S. Pat. No. 4,789,929 (1988). PA1 4. W. A. Kalandar, P. Vock and W. Seissler in Advances in CT (Springer-Verlag, Berlin, Heidelberg 1990, pp. 55-64). PA1 5. C. R. Crawford and H. F. King, Med. Phys. 1796), (1990) pp. 967-982 and reference therein. PA1 1. Linear and non-linear interpolation schemes; PA1 2. Variable bed speeds associated with appropriate interpolation schemes; and PA1 3. Reducing the bed speed so that the bed moves less than a whole slice width within a single gantry revolution. PA1 4. Reconstruction using data from only 180.degree. of rotation before the desired position and 180.degree. of rotation after the desired position. PA1 a gantry, PA1 a bed for supporting a scanned subject, PA1 multiple X-ray sources mounted on said gantry, PA1 an X-ray detector array on a side of the subject opposite to each of the X-ray sources, PA1 said detector arrays comprising detector elements for simultaneously detecting X-rays from multiple sources that have traversed one or multiple planar sections of said subject to acquire radiation density data, PA1 rotary drive mechanisms for rotating the X-ray sources about the subject, the detector arrays may be rotating opposite to the X-ray sources (3rd generation CT scanner) or stationary, (4th generation CT scanner), PA1 longitudinal drive mechanisms for causing relative motion in an axial direction between the bed and the gantry while the X-ray sources are revolving about the subject, and PA1 an image processor for reconstructing images from said data where said image processor includes a reformatting arrangement for reformatting the acquired data into single plane data by interpolating between data acquired by detector elements that may be exposed to different X-ray sources and/or to the same X-ray source.
Steps 2-4 are repeated as long as more slices are required. Step 3 may be concurrent with steps 2 and 4, but step 4 must be successive to step 2. Step 4 involves acceleration and de-acceleration of the bed if the bed is stationary during the scan such as when successive planar images are acquired. Step 2 may involve acceleration and de-acceleration of the gantry to the proper rotational speed. Gantry acceleration and de-acceleration may, however, be circumvented by using a continuous rotation scanner such as provided, e.g., by slip-ring technology.
An ubiquitous problem encountered by CT systems is that a complete 360.degree. rotation of the gantry is required to achieve a high quality image. At least 180.degree. of rotation is required to achieve a full set of projections. One solution is a multiple source X-ray tomograph such as described e.g. in U.S. Pat. No. 4,991,190 where high resolution or high speed scans may be achieved.
Other problems occur with the prior art CT scanners used to obtain a series of planar images. For example, the successive nature of the scanning process described hereinabove, prolongs the time during which the subject is imaged. The longer throughput time results in greater patient discomfort. The bed acceleration and de-acceleration add to the discomfort of the patient. Further, the patient is required to adjust his breathing cycle to the scanning rate so as to reduce motion related image artifacts. When the examination period is longer, the breath control is more difficult resulting in more patient motion, both during scans and between scans. Patient motion, voluntary and involuntary, between scans decreases the repeatability that is desired between adjacent slices. In particular, oblique reformatting and 3-D images formed from series of planar images are adversely affected.
To overcome these problems, helical or spiral scanning systems have been investigated and developed. This type of scanning is described in the following references:
Essentially, with helical scanning scanners, the subject is continuously scanned while the gantry makes multiple rotations about the subject and the bed is moved relative to the gantry along the axis of rotation simultaneously with the rotation. Images of successive slices are recontructed from sets of views using well known recontruction algorithms.
In conventional non-helical; i.e., stationary bed CT scans made to image successive slices, the different views making up the different sets correspond to projections within the same plane. On the other hand, in the helical scans the different views making up the different sets correspond to projections in different planes.
Therefore, non-modified conventional reconstruction yields artifacts; i.e., highly distorted images. To prevent such artifacts, the raw data is reformatted before backprojection into single plane data sets by interpolating between data measured at the same gantry angle but at different subject positions, providing data of different planes.
The theoretical slice sensitivity profile is defined as the response of the scanner to a small homogeneous object as a function of the object position along the axial direction. The slice width is defined as a full width at half maximum (FWHM) of the slice sensitivity profile.
In stationary-bed CT scans, the slice width is determined by collimators limiting the beam width or the length of the detector elements in the axial direction. In helical scans, data from different planes through the subject are mixed and the slice sensitivity profile is smeared. Therefore, the FWHM of the profile tends to be larger in a helical scan than in a stationary-bed scan for a given collimator setting. Also, the ratio between the full width at tenth maximum (FWTM) and the FWHM of the sensitivity profile, which as a measure of the quality of the slice width, is severely degraded.
Various schemes to improve the slice sensitivity profile in helican scans are discussed in the references cited hereinabove. These include:
Non of these schemes, however, provides images of the quality obtained in prior art stationary bed CT systems for a given radiation dose applied to the subject. Furthermore, because of the increased time length of exposure required in helical scans, the available X-ray intensity is likely to be less than in stationary bed CT systems, thus further decreasing image quality.
One possible solution to the problems encountered in prior art X-ray computed tomography is a multiple slice tomograph as taught in the above referred to Patent Application. The present invention provides both an alternative solution and an additional solution.